Ion exchange technologies are widely used for water and waste treatment, in areas such as hydrometallurgy, biochemistry, medicine and environmental protection. Ion exchange efficiency depends on many factors, the principal one being the selectivity of the exchanger in use. Inorganic ion exchangers and adsorbents, due to such properties as chemical and thermal stability, resistance to oxidation, and unique selectivity to certain ions, have definite advantages in comparison with traditionally used organic resins. Inorganic ion exchangers are able to operate in extreme conditions (high temperature or strong radiation fields, in the presence of organic solvents and/or oxidants and in a great excess of competitive ions), in which organic resins fail to work efficiently. Zirconium phosphate (ZrP) inorganic adsorbents have been studied in detail. Zirconium phosphates can be amorphous or crystalline. Zirconium phosphates, as generally known and defined herein, have the general formula ZrO2.nP2O5.mH2O.(xMO), where n=0-1.0, m>0, x=0-1.0, M=metal ion.
A conventional method of amorphous zirconium phosphate preparation includes reaction between aqueous solutions of a zirconium salt and phosphoric acid or its salts with formation of a gelatinous precipitate, filtering the precipitate, washing and drying. The final product after drying is a fine powder or granules with irregular form.
Depending on the experimental conditions (e.g., pH, temperature, duration) and composition of the reaction mixture, a P/Zr ratio in the final product can vary in a broad range from ˜0 up to 2.0. The presence of phosphorus-containing functional groups (e.g., HPO4, H2PO4) provides cation exchanger properties to zirconium phosphates. Some amorphous zirconium phosphates have a high affinity towards transition metals and heavy alkali ions. However, amorphous zirconium phosphates synthesized via such a precipitation route have several drawbacks which include:                strong dependency between ion exchange performance and moisture content, which suggests loss of capacity and deterioration of kinetics of sorption with the loss of water during storage or under drying;        low thermal stability; and        poor mechanical and hydrodynamic properties of the sorbents (powders, granules of irregular form), preventing use in column type applications.        
Amorphous zirconium phosphates in powdered form can be granulated with the use of organic or inorganic binders. This approach allows the production of mechanically strong ion exchangers in the form of beads or extrudates of desired shape suitable for column applications. However, use of binders affects total ion exchange capacity, kinetics of adsorption and makes some specific limitations on zirconium phosphate applications due to solubility of the binder and possibility of additional contamination of the product.
Granulated amorphous zirconium phosphates without binders can be prepared via sol-gel or gel routes. The sol-gel granulation process based on the oil-drop principle includes conversion of a ZrO2 sol into spherical granules of hydrous zirconium oxide gel in organic water-immiscible media, followed by conversion into zirconium phosphate by treatment of the ZrO2 gel with phosphoric acid or a phosphoric acid salt (R. Caletka, M. Tympl, J. Radioanal. Chem., 30: 155 (1976)). The gel method, also based on the oil-drop principle, may include reaction between aqueous solutions of zirconium salt and phosphoric acid (or its salt) in the presence of Zr-complexing reagent (H2O2, polyatomic alcohols, organic oxyacids) which allows a direct formation of zirconium phosphate gel (Amphlett, C. B. Inorganic Ion Exchangers. Elsevier, N.Y. (1964); Spherically granulated zirconium phosphate sorbents prepared via sol-gel and gel routes have high crush strength and good attrition resistance. However, they still have drawbacks of a strong dependency between ion exchange performance and moisture content, as well as low thermal stability.
A method of making granulated zirconium phosphate is described in U.S. Pat. No. 4,025,608. According to this method zirconium phosphate is made by the reaction of a zirconium salt, having a predetermined particle size, with phosphoric acid or a phosphate in a liquid medium. The zirconium phosphate made according to this patent has drawbacks of a strong dependency between ion exchange performance and moisture content, as well as low thermal stability.
Crystalline zirconium phosphates can be prepared by treatment of amorphous zirconium phosphates in the presence of excess H3PO4 at elevated temperature for a long period of time. (A. Clearfield, J. A. Stynes, J.Inorg.Nucl.Chem., v.26, 117, 1964) or by reaction between aqueous solutions of a zirconium salt and phosphoric acid to form a gel and then heating the dry gel in water under hydrothermal conditions (M. K. Dongare et al, Mat.Res.Bull. v.27, 637-645, 1992) and also via solid state reactions between ZrO2 or Zr salts and salts of phosphoric acid (J. M. Winand et al, J.Solid State Chem., 107, 356 (1993)).
Depending on the experimental conditions and the compositions used, various crystalline forms of zirconium phosphate, both layered and framework, have been reported. Among them are hydrated materials like α-Zr(HPO4)2 H2O (A. Clearfield, J. A. Stynes, J.Inorg.Nucl.Chem., v.26, 117, 1964), γ-Zr(H2PO4)(PO4) 2H2O (A. Clearfield et al, J.Inorg.Nucl.Chem., v.30, 2249, 1968), τ-Zr(HPO4)2 H2O (A. M. K. Andersen et al, Inorg.Chem., v.37, 876-881, 1998, ψ-Zr2O3(HPO4) nH2O (A. Clearfield et al, Inorg.Chem.Comm., 1, 208 (1998), HZr2(PO4)3 H2O (S. Feng, M. Greenblatt, Chem.Mater., v.4, 1257, 1992), or non-hydrated materials like MZr2(PO4)3, MZr5(PO4)7 (M. K. Dongare et al, Mat.Res.Bull. v.27, 637-645, 1992), M5Zr(PO4)3 (J. P. Boilot et al, J.Solid State Chem., 50, 91 (1983), ZrP2O7, Zr(OH)PO4 (N. G. Chernorukov et al, J.Inorg.Chem., v.28, 934 (1984). Some of the crystalline zirconium phosphates contain exchangeable ions (H+ or metal cations) and show ion exchange properties. The selectivity of crystalline materials strongly depends on the type of crystal structure and, in some cases, is much higher than that of amorphous compounds. Another advantage of crystalline materials is that they are less susceptible to moisture content than amorphous sorbents and, as result, are more thermally stable. Among disadvantages of crystalline ion exchangers are poor kinetics of adsorption and powdered form, preventing their use in column applications.